Spread spectrum method

ABSTRACT

A method for establishing and communicating synchronous, code division multiple access communications between a base station and a plurality of remote units. A plurality of remote-communications signals which have spread-spectrum modulation are transmitted from the plurality of remote units and arrive simultaneously at the base station. The method includes transmitting repetitively from the base station a access signal having spread-spectrum modulation and receiving the access signal at an accessing-remote unit. The accessing-remote unit transmits an echo signal which has spread-spectrum modulation. The echo signal is received at the base station and a time delay is measured between the access signal and the received echo signal. A protocol signal which has spread-spectrum modulation is transmitted to the accessing-remote unit, with the protocol signal communicating a chip codeword and the time delay measured at the base station. The accessing-remote unit adjusts a delay time such that the communications signal transmitted from the accessing-remote unit arrives simultaneously at the base station with communications signals transmitted from the plurality of remote units. The base station communicates to the plurality of remote units, with a plurality of base-communications signals. Each chip codeword of each base-communications signal is orthogonal to chip codewords of the plurality of base-communications signals. 
     The plurality of remote-communications signals arrive simultaneously at the base station with each chip codeword of each remote-communications signal orthogonal to chip codewords of the plurality of remote-communications signals.

RELATED PATENTS

This application is a continuation of application Ser. No. 07/703,095,filed May 22, 1991 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a code division multiple access (CDMA) spreadspectrum system, and more particularly to a method for establishingcommunications in a personal communications network.

DESCRIPTION OF THE RELEVANT ART

While prior art CDMA spread spectrum systems, such as one beingdeveloped by QUALCOM, use time synchronization, the design is not verysensitive to how accurately this is done. Basically the chip codewordsare randomized so that the radio receiving the transmitted signal usingchip code, x, experiences the equivalent noise variance, N₁ due to M-1interference signals as

    N.sub.1 =(M-1)L

and the total sum of such noise variance terms among all radios is##EQU1##

For the case M=L this results in non-zero interference compared to theuse of orthogonal codewords which give zero interference. Such a priorart system clearly is not optimum for the case where M=L. For M>>L,however, the difference between the prior art system interference andthe lower bound by Welch (1974) is quite small.

In their cellular radio system QUALCOM assumes that the number of usersM is much larger than the code length L and to overcome the interferencethey use strong error correction coding that gives 5 dB of coding gainand voice activation which accounts for an additional 5 dB. Togetherthis is about 10 dB of gain.

Because the cellular cells are quite large, there are typicallymultipath signals with delay differences larger than a chip timeinterval. QUALCOM apparently uses an adaptive RAKE type receiver thatcoherently combines multipath components. This improves the overallperformance for the cellular radios.

OBJECTS OF THE INVENTION

A general object of the invention is a high capacity microcell usingspread spectrum modulation.

An object of the present invention is a method for establishingcommunications using spread spectrum modulation in a personalcommunications network.

Another object of the present invention is a CDMA spread spectrumcommunications system where all signals from mobile units arrive at abase station within a fraction of a chip time interval.

A further object of the present invention is a CDMA spread spectrumcommunications system which uses orthogonal chip codes which have across correlation of zero.

SUMMARY OF THE INVENTION

According to the present invention, as embodied and broadly describedherein, a method is provided for establishing and communicatingsynchronous, code division multiple access communications between a basestation and a plurality of remote units. A plurality ofremote-communications signals which are modulated with spread-spectrumand which use the same carrier frequency as the base station, istransmitted from the plurality of remote units and arrive simultaneouslywithin a fraction of a chip time interval at the base station.

The method for establishing communications for each remote unitcomprises the steps of transmitting repetitively from the base stationan access signal which is modulated with spread-spectrum. The accesssignal is received at an accessing-remote unit. The accessing-remoteunit is a remote unit which desires to establish communications with thebase station. After receiving the access signal, the accessing-remoteunit transmits an echo signal which is modulated with spread-spectrum.The base station receives the echo signal and measures a time delay fromwhen the access signal was transmitted and the echo signal is received.

A protocol signal, which is modulated with spread-spectrum, istransmitted from the base station to the accessing-remote unit. Theprotocol signal communicates to the accessing-remote unit a chipcodeword and the time delay measured at the base station. Upon receivingthe protocol signal at the accessing-remote unit, the accessing-remoteunit adjusts a delay time such that the communications signaltransmitted from the accessing-remote unit arrives simultaneously at thebase station with communications signals transmitted from the pluralityof remote units. The delay time is adjusted relative to the accesssignal which is repetitively transmitted from the base station. The chipcodeword is used by the accessing-remote unit for communicating with thebase station.

The base station communicates to the plurality of remote units with aplurality of base-communications signals which are modulated withspread-spectrum and transmitted simultaneously and on the same carrierfrequency from the base station. Each of the base-communications signalshas its own unique chip codeword. Each chip codeword of eachbase-communications signal is orthogonal to other chip codewords used bythe plurality of base-communications signals.

The plurality of remote units communicates to the base station with theplurality of remote-communications signals, respectively, which aremodulated with spread-spectrum. The plurality of remote-communicationssignals, which use the same carrier frequency, are transmitted from theplurality of remote units, respectively, so that the plurality ofremote-communications signals arrive simultaneously at the base station.Each chip codeword of each remote-communications signal is orthogonal tochip codewords of the plurality of remote-communications signals. Eachof the remote-communications signals has its own unique chip codeword.For a particular two-way communications channel between a particularmobile using and the base station, the unique chip codeword used for thebase-communications signal and the remote-communications signal,respectively, may be the same.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention also may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute apart of this specification, illustrate particular embodiments of theinvention, and together with the description, serve to explain theprinciples of the invention.

FIG. 1 illustrates a base station and a plurality of remote unit;

FIG. 2 illustrates the sequence of signals of the present invention;

FIG. 3 shows a maximal length shift register; and

FIG. 4 illustrates a hand off geometry which can be used with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention disclosed in the patent application is related to U.S.patent application Ser. No. 07/528,020, filed on May 24, 1990 by J. K.Omura and D. Avidor, and entitled "METHOD AND APPARATUS FOR MODULATIONAND DEMODULATION OF SPREAD SPECTRUM SIGNALS," which is incorporatedherein by reference.

Reference will now be made in detail to the present preferredembodiments of the invention, an example of which is illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

A preferred embodiment of the present invention, as shown in FIG. 1,includes a base station 110 and a plurality of remote units,illustrated, by way of example, as first, second, and third remote units111, 112, 113. The base station 110 includes spread spectrumtransmitters and receivers, and appropriate processors for implementingthe methods of the present invention. Each of the plurality of remoteunits 111, 112, 113 includes a spread spectrum transmitter, a spreadspectrum receiver and an appropriate processor. With current technology,a digital signal processor may implement the method of the presentinvention at each remote unit. A remote unit may be embodied as awireless telephone, radio paging device, computer terminal, or any othervoice, video or data device with appropriate circuitry for implementingthe present invention.

As a preferred embodiment, the present invention can use the 902 MHz to928 MHz ISM band where there are 10 non-overlapping frequency sub-bandsof 2.6 MHz each. Each of the 2.6 MHz bands can have up to 16simultaneous spread spectrum signals where each uses a differentorthogonal spreading chip sequence of 16 chips. In this example, timedivision duplexing, (TDD) is used to achieve full duplex 32 Kbps datarate in both direction on the same carrier frequency. The presentinvention illustrates how spread spectrum radios are used to make a highcapacity microcell and how a collection of such microcells worktogether.

Typically, in a frequency division multiple access (FDMA) microcell thespread spectrum radios use several non-overlapping sub-bands, but theentire system overlays on the total available frequency band. In the ISMband from 902 MHz to 928 MHz, for example, there is a total bandwidth of26 MHz. With direct sequence/binary phase shift keying (DS/BPSK)personal communication service (PCS) radios, which each require 2.6 MHz,10 non-overlapping full duplex FDMA spread spectrum radio channels areavailable in this band. Thus, several non-interfering channels exist dueto frequency separation. Where there is simultaneous use of the samefrequency in the microcell, a code division multiple access (CDMA)system uses orthogonal chip sequences. Thus we have an FDHA overlay onCDMA channels.

The present invention for a microcell has several unique features whichare designed to allow more radios per microcell and to minimizeinterference among the radios in a collection of microcells. The resultscan readily be generalized.

The Spread Spectrum Digital Radio

The present invention uses conventional direct sequence binary phaseshift keyed (DS/BPSK) modulation with a fixed spreading sequence usedfor each data bit. This is the best known and most widely usedcommercial spread spectrum radio modulation. There are many ways,however, to design the receiver for DS/BPSK radios.

One implementation of the basic radio can use 32 kbps ADPCM to digitizevoice in a PCS application. As an example, 16 chips per bit can be usedas the spreading sequence resulting in 512 chips per second. With theuse of time division duplexing (TDD) and some time guard band betweenbursts, the bandwidth of each radio signal is 2.6 MHz.

One unique feature of the receiver for DS/BPSK is the use of digitalmatched filters implemented in the manner described in the U.S. patentapplication Ser. No. 07/528,020. For the DS/BPSK system 8-bit samplesare most likely used with five samples per chip. Alternatively, a deltamodulation variation of this digital matched filter can be used asdescribed in the paper "A Proposal of New Detector Scheme Using DeltaModulation Type Digital Matched Filter for Direct-SequenceSpread-Spectrum Communication System" by Hisao Tachika and TadashiFujino of Mitsubishi Electric Corporation presented at the 1990International Symposium on Information Theory and Its Applications inHawaii on Nov. 27-30, 1990.

Another feature is the open loop type phase and frequency trackingmodule which acquires and tracks the phase and frequency of the receivedsignal. The phase and frequency tracking module allows the receiver toadjust its own oscillator to match the frequency of the received signal.In the microcell system of the present invention, the base stationradios serve as master radios, and are assumed to have a very accuratefrequency and time reference. The remote units, which may be embodied asmobile telephone radios, are slave to the corresponding base stationmaster radios. The slave radios lock onto the master radio carrierfrequency and use the master radio signal for establishing its timereference.

The present invention uses time division duplexing (TDD), also calledthe "ping pong" protocol, to achieve full duplex operation using thesame frequency. This is an old idea used in some modems and it iscurrently used in the new cordless telephones, denoted CT-2, in England.TDD reduces the radio complexity by using only one radio frequencysection for both transmission and reception rater than the method ofusing two frequency bands, one for transmission and one for reception,as used in cellular radios.

With TDD the master radio at the base station controls all timing withthe corresponding slave radio in the remote unit. The slave radio at theremote unit transmits its bursts at a fixed delay from the end of themaster radio's bursts. The fixed time delay is determined by the masterradio.

The invention includes a method for establishing and communicatingsynchronous, code division multiple access (CDMA) communications betweenthe base station 110 and the plurality of remote units. A plurality ofremote-communications signals which are modulated with spread-spectrumand have the same carrier frequency, is assumed to be transmitted fromeach of the plurality of remote units. A remote-communications signalcommunicates information from each remote unit to the base station 110.The timing for transmitting the remote-communications signals isadjusted so that all the remote-communication signals arrivesimultaneously at the base station 110. Each of the plurality of remoteunits initially was set in proper synchronization prior to communicatingwith the base station 110.

As illustratively shown in FIG. 2, the method for establishingcommunications and setting proper synchronization for each remote unitcomprises the steps of transmitting repetitively from the base station110 an access signal which is modulated with spread-spectrum. The accesssignal may use the same or a different carrier frequency from that usedby the plurality of remote units. The carrier frequency and chipcodeword of the access signal are known to all remote units which cancommunicate with the base station 110. Each remote unit may have areceiver dedicated for listening to and receiving the access signal.Alternatively, each remote unit may have a receiver which is setinitially to listen to the access signal, and which adapts to adifferent chip codeword after communications is established with thebase station.

A chip codeword generator, by way of example, can have shift registerswith taps set to match the spread spectrum chip codeword of the accesssignal. The taps of the shift registers can be reset to match a spreadspectrum chip codeword of a remote-communications signal or abase-communications signal.

The access signal is received at an accessing-remote unit 114. Theaccessing-remote unit 114 is the remote unit desiring to establishcommunications with the base station 110. After receiving the accesssignal, the accessing-remote unit 114 transmits an echo signal which ismodulated with spread-spectrum. The base station 110 receives the echosignal and measures a time delay from when the access signal wastransmitted and the echo signal is received. The echo signal may use thesame chip codeword as was used by the access signal, or a different chipcodeword. Typically, the same chip codeword is used for establishingcommunications with the base station 110.

A protocol signal, which is modulated with spread-spectrum, istransmitted from the base station 110 to the accessing-remote unit 114.The protocol signal communicates to the accessing-remote unit 114 aremote-chip codeword and the time delay measured at the base station.The protocol signal may use the same chip codeword as the access signal.The remote-chip codeword may be communicated by a codeword embedded inthe protocol signal, which sets taps on shift registers in a chip-codegenerator in the accessing-remote unit 114, which generates theremote-chip codeword. Chip codes may also be stored in memory of theradios. The remote-chip codeword is used by the accessing-remote unitfor communicating with the base station 110.

The time delay is measured from when the access signal was transmittedfrom the base station 110, to when the echo signal was received at thebase station 110. The time delay is proportional to the distance theaccessing-remote unit 114 is located from the base station 110.

Upon receiving the protocol signal at the accessing-remote unit 114, theaccessing-remote unit 114 adjusts a delay time such that thecommunications signal transmitted from the accessing-remote unit 114arrives simultaneously in time at the base station 110 withcommunications signals transmitted from the plurality of remote units.The delay time is adjusted relative to the access signal which isrepetitively transmitted from the base station 110.

The base station 110 communicates to the plurality of remote units witha plurality of base-communications signals which are modulated withspread-spectrum and transmitted simultaneously and on the same carrierfrequency from the base station 114. The plurality of base-communicationsignals are transmitted simultaneously in time with the access signal.Each of the base-communications signals has its own unique chipcodeword. Each unique chip codeword of each base-communications signalis orthogonal to chip codewords used by the plurality ofbase-communications signal, and the access signal.

The plurality of remote units communicates to the base station 110 withthe plurality of remote-communications signals which are modulated withspread-spectrum. The plurality of remote-communications signals, whichuse the same carrier frequency, are transmitted from the plurality ofremote units, respectively, so that the plurality ofremote-communications signals arrive simultaneously in time at the basestation 110. The delay time for the remote-communications signal,transmitted from a particular remote unit, was set when the particularremote unit initially accessed and established communications with thebase station 110. Thus, each remote unit has its own delay time, and ismeasured with respect to when the remote unit receives the accesssignal.

Each chip codeword of each remote-communications signal is orthogonal tochip codewords of the plurality of remote-communications signals. Eachof the remote-communications signals has its own unique chip codeword.For a particular two-way communications channel between a particularremote unit and the base station 110, the unique chip codeword used forthe base-communications signal and the remote-communications signal maybe identical.

Thus, the base station 110 radios are timed together so that they sendout their TDD bursts at the same time. Also, each remote unit, as aslave radio, receives all of the base-communications signals arrivingtogether and separates the desired base-communications signal using itsunique orthogonal chip sequence. Since the plurality ofbase-communications signals arrive at any particular remote unit at thesame time, the spread spectrum receivers and processor at the particularremote unit can use orthogonal chip sequences to separate out thevarious signals from the plurality of base-communications signals, whichare transmitted from the base station 110, to the plurality of remoteunits.

During initial acquisition an accessing-remote unit 114 can search forand acquire the base-communications signal with the chip sequencecorresponding to an access channel. Once a slave radio of anaccessing-remote unit 114 acquires the master radio of an accesschannel, it can begin the access protocol which includes the processorat the base station 110 determining the propagation time delay due tothe distance of the accessing-remote unit 114 from the base station 110.The processor at the base station 110 assigns a frequency, code, andtime delay to the accessing-remote unit 114 during this access protocol.The time delay is determined by the processor when it determines thepropagation time delay between an accessing-remote unit and the basestation 110, using well known ranging or echo techniques. Thus, theslave radios of the plurality of remote units have their transmissionbursts timed to arrive at the base station within a fraction of a chiptime interval. With thistiming, the spread spectrum receivers andprocessor at the base station 110 can use orthogonal chip sequences toseparate out the various spread spectrum signals from the plurality ofremote-communications signals, which are transmitted from the pluralityof remote units to the base station 110.

With TDD, the master radios at the base station 110, which have accuratefrequency and time references, can tightly control all the slave radiosof the remote units that are in the microcell. A slave radio locks ontothe carrier frequency of one of the master radios and transmit its TDDbursts at some fixed time delay relative to the end of the masterradio's TDD bursts.

The tight control that the microcell base station master radios has overthe telephones slave radios of the remote units results in a system thatcan take advantage of orthogonal chip sequences to achieve higheroverall capacity. Capacity is the number of active mobile telephonechannels that can be accommodated by a microcell.

Continuous Update of Delay

The present invention also provides for continuously updating the timedelay measured at the base station. Using a base station receiver, anoutput peak of the cross correlation of a receivedremote-communications-signal from a remote unit provides timing fortracking the time delay. The time measured by the cross correlation canbe used to adjust the delay time such that the communication signaltransmitted from the remote unit continuously arrives simultaneously atthe base station with communications signals transmitted from theplurality of remote units, while the moving-remote unit moves a way fromor toward the base station. The time delay can be measured by crosscorrelating the codeword of the moving-remote unit with a stored replicaof the codeword at the base station. Accordingly, the delay time can beupdated within a fraction of a chip interval, thereby providing accuratetiming between the moving-remote unit and the base station.

Orthogonal Chip Sequences

To allow a large number or active radios in each microcell, anorthogonal set of spreading chip sequences is used. For chip sequencesof 16 chips per bit, up to 16 radios can use one of the FDMA channels solong as all the radios are time synchronized such that each of the 16radios have their spreading code sequences timed to arrive at the masterradios at the base station of the microcell within a fraction of a chiptime interval. This critical time synchronization allows orthogonalspreading sequences to be used so that the 16 radios will not interferewith each other.

Sixteen orthogonal spreading sequences of 16 chips in length isillustrated with the Hadamard matrices. Let ##EQU2## for n=2, 3, 4. Withthis construction there are 16 rows of H(4) that are orthogonalsequences of length 16. Another way to obtain a set of orthogonal chipsequences of 16 chips in length is to use a maximal length sequence ofperiod 15. Let c be any one period of the sequence where the values are+1 or -1 and T^(n) c be the n cycle shift of c. The cross correlation is##EQU3## A set of 16 orthogonal chip sequences of 16 chips in length isgiven by adding a "1" component to each of the 15 shifts of c and thevector of 16 "1s".

By synchronizing in time all the slave radios of the plurality of remoteunits it is possible to have 16 non-interfering wireless communicationlinks for each of the 10 FDMA channels resulting in a total capacity of160 wireless links per microcell. In this system the remote units, suchas telephone radios, time their transmissions so that they all arrive atthe base station synchronized to within a fraction of a chip timeinterval. These remote units also are locked in frequency to theircorresponding base station master radios.

Access Channel

Two ways are described to achieve an access channel where aaccessing-remote unit can initially synchronize its radio frequency andtime reference and conduct communication before it places an outgoingcall or while the accessing-remote unit is in a standby mode, a basestation radio as a master radio, controls the average power of themobile telephone slave radio, it is controlling.

Each base station radio is assumed to have two antennas for diversity.Besides keeping the time average power of the slave radio at some fixedlevel, if a short time drop in the power level is received from theslave radio of a remote unit then the master radio of the base station110 switches to the alternate antenna. Power control is assumed to varyslowly and based on a long time average while the fading due to theslave radio passing through multipath nulls is handled by the antennaswitching by the master radio.

Each slave radio has only one antenna. Antenna diversity is only done bythe master radio using its two antennas.

Spread Spectrum CDMA Code Selection

High capacity PCN systems using spread spectrum radios requires carefulchoice of spread spectrum chip codewords and system timesynchronization. A single carrier frequency in a single microcell formsa star network with base station 110 and remote units 111, 112, 113,114. The radios all use spread DS/BPSK spectrum signals and TDD toachieve full duplex operation. Assume that the TDD bursts aresynchronized among a plurality of base station radios which serve asmaster radios, to their corresponding remote units, which are slaveradios. Here each master radio is paired with one slave radio to form a32 kbps ADPCM voice channel.

Assume that there are M active slave radios transmitting simultaneouslyon one carrier frequency to the microcell base station master radios.Thus, a M radio signals are in the channel at any given time. There arealternating synchronized burst transmissions from the M slave radios andthe M master radios.

The Welch Bound

Each spread spectrum radio has a chip codeword of length L. Supposethere are M radios where the Mth radio has a chip codeword denoted

    x.sub.m =[x.sub.m1, x.sub.m2, x.sub.m3, . . . , x.sub.mL ]

where the kth chip, x_(mk), is either -1 or 1. The set of M chipcodewords from a code denoted

    {x.sub.1, x.sub.2, x.sub.3, . . . , x.sub.M }

The cross correlation between the chip codewords determine the amount ofinterference any receiver has when there are M radios that aresimultaneously transmitting in the same band. For example, the receiverthat is trying to receive the transmitting radio using the first chipcodeword, x₁, has an interference term from the transmitting radio usingthe second chip codeword, x₂, given by the cross correlation ##EQU4##

The total variance of all such interference terms gives a measure of theequivalent noise variance which to this receiver is ##EQU5##

Lloyd Welch of USC derived a bound on how small this total interferencecan be with any M chip codewords of length L. He found that the sum ofall such equivalent noise variance terms is lower bounded by, ##EQU6##

Note that for M=L the lower bound is zero. If orthogonal chip codewordswere used then all the cross correlations would be zero and the lowerbound would be achieved. In general it is possible to find M=Lorthogonal chip sequences of length L.

Since the Welch bound is greater than zero for M>L this tells us that itis impossible to find more than L orthogonal chip sequences of length L.

System Time Synchronization

The Welch bound was derived for the special case where at any givenreceiver all the transmitted spread spectrum signals, the intendedsignal plus the M-1 interfering signals, arrive at the receiver timesynchronized so that the chip codewords are lined up in time.

Since the master radios of the base station 110 are located together itis easy to time synchronize their transmissions so that all slave radiosreceive all M transmitted signals with their chip codewords arriving atthe same time.

To synchronize the remotely scattered slave radios each master radiomust control its corresponding slave radio so that all slave radiosignals are time synchronized at the base station. Under this assumptionthe Welch bound applies.

Without time synchronization at the chip time level, it is not possibleto get the kind of small interference suggested by the Welch lowerbound. If the number of users M gets much larger that the chip codelength L then the difference between having system time synchronizationand not having it becomes small.

Chip Codes

One set of chip codes is described that has near optimal properties andseems to be particularly well suited for multiple microcell PCNapplications.

A five register maximal length sequence generator with feedback tapsfrom registers [3, 5] shown in FIG. 3 can be used to generate a binarysequence of length 31 which as some important properties.

Let s be the 31 bit sequence that is generated with the initial fiveregister values of 11111. This sequence is given by

    s=[1111100011011101010000100101100]

and a cycle shift operator T on s is given by,

    Ts=[111100011011101010000100101100]

All the 31 distinct cycle shifted sequences are T^(k) s for k=0, 1, 2, .. . , 30.

Suppose that each of the 31 cycle shifted sequences are concatenatedwith a "0" to it and make a binary sequence of 32 bits in length andconvert them into 31 chip codewords of 32 chips in length by changingeach "0" bit "1" amplitude and each "1" bit to "-1" amplitude. Let the31 chip code be denoted

    {x.sub.1, x.sub.2, x.sub.3, . . . , x.sub.31 }

Note that if chip codeword x₂ was constructed form Ts then

    x.sub.2 =[-1, -1, -1, -1, 1, 1, 1, -1, -1, 1, -1, -1, -1, 1, -1, 1, -1, 1, 1, 1, 1, -1, 1, 1, -1, 1, -1, -1, 1, 1, -1, 1]

The interesting property of these converted maximal length sequences isthat they are orthogonal. That is,

    <x.sub.m, x.sub.n >=0 when m≠n

There are a total of six maximal length sequences generated by fiveregister generators. Their feedback connections are given by [2,5],[4,5], [1,2,3,5], [1,2,4,5], [1,3,4,5] and [2,3,4,5].

Each of the six maximal length sequences of length 31 can form a chipcode of 31 orthogonal codewords where each such codeword is 32 chipslong. Thus, there are six codes consisting of a set of 31 orthogonalcodewords. The cross correlation of codewords from different codes,however, are not necessarily orthogonal.

Because of the shifting property, each codeword from one code has thesame set of cross correlation values when it is cross correlated withall the codewords of another chip code. In fact, if y is a codeword fromone chip code and {x₁, x₂, x₃, . . . , x₃₁ } is a set of codewords fromany other chip code then the equivalent noise variance is ##EQU7##

Handoff From Microcell to Microcell

Suppose two adjacent microcells are being used where within a microcellthe radios use orthogonal chip sequences. Orthogonality between any chipsequence of one microcell and a chip sequence from the other microcellis assumed not necessary.

In terms of timing between the two microcells, assume that bothmicrocells have their TDD bursts synchronized to within a chip timeinterval. This synchronization can be maintained using a single radiochannel between the two microcells making use of directional antennasbetween the two base stations.

Two basic ways are describe to allow a mobile telephone to be handed offfrom one microcell to another as the transitioning-remote unit roamsaround in an area covered by the two microcells.

The first technique assumes that each microcell base station has anextra set of radios that are searching for signals from mobiletelephones that are linked to the other microcell. Since the twomicrocells use different set of orthogonal spreading chip sequences, theextra radios in one microcell can be searching for signals using thespreading chip sequences of the other microcell. If such a signal isdetected and its signal strength exceeds a threshold, then this wouldindicate that the transitioning-remote unit is getting close to the basestation of the microcell that has detected it. This microcell caninitiate handoff of this mobile telephone from the other microcell toitself by using the radio link that connects the base stations of thetwo microcells. Since the base station radios control the mobiletelephones through an in-band channel, the hand off of the mobiletelephone from one microcell to another can be easily handled by thebase stations of the two microcells.

This method requires that each microcell has extra radios at its basestation that are searching for strong signals from mobile telephonesthat are active and linked to adjacent microcells. This is possiblebecause each microcell is assumed to use a unique set of orthogonal chipsequences.

Rather than have extra radios at base stations of microcells, eachmobile telephone unit can have a second receiver that searches forsignals from adjacent microcells while it has an active call in session.This can be achieved with only the digital base band portion of thereceiver if this search is to be limited to the same frequency as iscurrently being used by the active session.

The second method of establishing connection to a new microcell placesthe responsibility of finding a new microcell during roaming on thetransitioning-mobile unit. This is similar to the initiation of handoffused by the receiver.

Each microcell is assumed to have a known unique set of spreading chipsequences. Each microcell may use a unique chip sequence for its accesschannels. Thus, any mobile telephone unit can search for differentmicrocell access channels by using the unique chip sequences.

Performance With Two Microcells

Consider two adjacent microcells are denoted "A" and "B" where microcellA uses code {x₁, x₂, x₃, . . . , x₃₁ } and microcell B uses another code{y₁, y₂, y₃, . . . , y₃₁ }. A remote unit is assumed to have slave radioestablished a voice link with microcell A's base station using codewordx₁. While it is receiving the TDD bursts from this base station it isalso receiving interference signals from microcell B's base station. Theperformance of this radio is examined with this interference when thetwo codes used are the type described herein.

Let P_(A) and P_(B) be the power of the first base station and thesecond base station, respectively, at the radio using code x₁. For thiscase where the variance of the cross correlation terms is ##EQU8## andthe signal term is (32)² =1024, the equivalent bit energy to noise ratiois given by

    E.sub.b /N.sub.o =P.sub.A /P.sub.B

Suppose a rate 1/2 convolutional code with constraint length K=7 isused, where each information bit is coded into two bits that form the4-ary phase of a DS/QPSK system. For this system, the 32 chip spreadingcodes given above are used. A bit error probability of 10⁻³ requires##EQU9##

This can be achieved so long as

    P.sub.A ≧2P.sub.B

If in addition all radios use voice activation circuits where thetransmit power is turned off during silence in a voice conversation thenon the average the interference is reduced to 35%. Thus a remote unithas a good voice communication channel even when the TDD bursts from thetwo base stations are of equal power at its location. In this case theremote unit is actually receiving 31 bursts signals from microcell A'sbase station. Since it is using one of the orthogonal 31 codewords, x₁,it does not get any interference from this microcell. The remote unit isalso receiving 31 interference signals from microcell B's base stationwhere these signals have codewords that are not orthogonal to x₁ andthus add equivalent noise to its channel. The boundary of all locationsfor the slave radio where its bit error rate is less than 10⁻³approximately corresponds to the location where the power of the twomicrocell base station radios are equal.

Different Power Levels for each FDMA Channel

The 902-928 MHz band can have 10 FDMA channels each of 2.6 MHz ofbandwidth. In each of these 2.6 MHz bandwidths channels spread spectrumsignals are used with up to 16 simultaneous active calls. This is wherethe code length L=16 chips for 32 Kbps full duplex voice using TDD. Forcodes of length L=32 there are 5 channels of 5.2 MHz of bandwidth each.

Note that each of the 10 FDMA channels form an independent microcellwhose range or radius is determined by the power of all radios usingthat particular carrier. Suppose, for example, that the carrierfrequency of the 10 FDMA channels are denoted f₁, f₂, . . . , f₁₀ wherethese are equally spaced in the 902-908 MHz band. Then one microcell mayuse power levels P₁, P₂, . . . , P₁₀ for the different carriers and thusthis microcell has different range radius for each of the different FDMAchannels. Another microcell may use different power distribution for the10 channels and thus it will have a different range radius for each ofits 10 FDMA channels.

The use of different power levels for the different non-overlappingspread spectrum frequency bands will allow a greater flexibility ofcoverage of mobile telephones in a given area and facilitate handofffrom one microcell to another. Consider two microcells in FIG. 4 wherecontours of a fixed power level to a slave radio are illustrated.

Suppose microcell A uses low power in carrier f₁ while microcell B usesa much greater power for this same carrier. Then as shown in FIG. 4microcell B has a much greater area coverage than microcell A. Nextsuppose that the for carrier f₂ the opposite is true. That is, microcellA uses much greater power at f₂ while microcell B uses little power atf₁. This is also shown in FIG. 4. This shows that a mobile telephone canalways be covered by at least one band of one of the microcell whilethere is little interference between the signals coming from the twomicrocell to the remote unit since in a single band in the cell thefixed power level areas need not overlap.

Note that handoff of a mobile telephone from one microcell to anothercan be done more easily by merely switching frequencies. For example,suppose that a remote unit is at location 55 shown in the FIG. 4 and itis using carrier f₁ to link with microcell B. Note that it is notgetting much interference from the same frequency channel of microcellA. As it roams and moves out of range of the f₁ carrier of microcell B,the remote unit can be handed off to microcell A using carrier f₂. Afterthis switch to carrier f₂ of microcell A, the mobile telephone getslittle interference from microcell B. This concept can, of course,easily be generalized to many such FDMA channels.

Generalization to Multiple Microcells

Suppose that five microcells, denoted "A", "B", "C", "D" and "E", andeach of these microcells use a different orthogonal code of 31 codewordseach of 32 chips in length. These are five of the six codes describedabove.

In the area covered by the five microcells, for a given carrierfrequency, there are a total of 5×31=155 active radio channels. Supposefor the moment that all five microcell base station radios have equalpower at the receiving remote unit. Then the bit energy to noise ratiofor the QUALCOM system is

    E.sub.b /N.sub.o =0.42=-3.81 dB

while the Welch lower bound gives

    E.sub.b /N.sub.o ≦0.52=-2.87 dB

The present invention described here has under the same condition thebit energy to noise ratio of

    E.sub.b /N.sub.o =0.50=-3.01 dB

The more general situation is where any receiving remote unit seesdifferent power levels from the five microcell base station radios. LetP_(A), P_(B), P_(C), and P_(D) be the power levels at the slave radio ofthe remote unit. If the remote unit has an active link with microcell Athen it does not receive any interference from this microcell's basestation and the equivalent bit energy to noise ratio is

    E.sub.b /N.sub.o =P.sub.A /(P.sub.B +P.sub.C +P.sub.D +P.sub.E)

Recall that for a coded DS/QPSK the required bit energy to noise ratioto achieve a bit error rate of 10⁻³ is E_(b) /N_(o) =2. Thus, if we usevoice activation circuits then for this remote unit to maintain a goodcommunication channel with microcell A it must have the condition

    P.sub.A ≧P.sub.B +P.sub.C +P.sub.D +P.sub.E

This result applies to one carrier frequency. At another carrierfrequency the power distribution of the five microcell base stations atthe slave radio of the remote unit may be quite different. By usingdifferent power levels for each FDMA channel by each base station, allareas covered by the five microcells can have adequate power levels toachieve a good communication channel with at least one microcell at somecarrier frequency.

In a radio network with many potentially interfering radiossimultaneously transmitting in the same band, interference to any givenradio is greatest from those with the most power at the receivingantenna of the radio. With equal power transmitters this usually meansthat the greatest interference comes from transmitters that are closestto the receiving radio. In the prior art all radios contribute the samecorrelation value while in the present invention the radios in the samemicrocell use orthogonal codes. Thus, the closest radios tend to havezero cross correlation and contribute no interference. Overall thisresults in a higher capacity system.

It will be apparent to those skilled in the art that variousmodifications can be made to the spread spectrum method of the instantinvention without departing from the scope or spirit of the invention,and it is intended that the present invention cover modifications andvariations of the spread spectrum method provided they come within thescope of the appended claims and their equivalents. Further, it isintended that the present invention cover present and new applicationsof the method of the present invention.

I claim:
 1. A method for establishing and communicating synchronous,code division multiple access communications on a same frequency betweena base station and a plurality of remote units, with a plurality ofremote-communications signals having spread-spectrum modulation from theplurality of remote units arriving simultaneously at said base station,the method for each accessing-remote unit comprising the stepsof:transmitting repetitively from said base station an access signalhaving spread-spectrum modulation; receiving the access signal at saidaccess-remote unit; transmitting responsive to the access signal, fromsaid accessing-remote unit, an echo signal having spread-spectrummodulation; receiving the echo signal at said base station; measuring atime delay at said base station between the access signal and thereceived echo signal; transmitting a protocol signal havingspread-spectrum modulation from said base station to saidaccessing-remote unit, with the protocol signal communicating a chipcodeword and the time delay measured at said base station; receiving theprotocol signal at said accessing-remote unit; adjusting at saidaccessing-remote unit a delay time such that the communications signaltransmitted from said accessing-remote unit arrives simultaneously atsaid base station with communications signals transmitted from theplurality of remote units; communicating from said base station to saidplurality of remote units, with a plurality of base-communicationssignals having spread-spectrum modulation transmitted simultaneouslyfrom said base station, with each chip codeword of eachbase-communications signal orthogonal to chip codewords of the pluralityof base-communications signals; and communicating from said plurality ofremote units to said base station with the plurality ofremote-communications signals having spread-spectrum modulationtransmitted from said plurality of remote units, respectively, theplurality of remote-communications signals arriving simultaneously atsaid base station with each chip codeword of each remote-communicationssignal orthogonal to chip codewords of the plurality ofremote-communications signals.
 2. A method for communicatingsynchronous, code division multiple access communications between a basestation and a plurality of remote units, with a plurality ofremote-communication signals having spread-spectrum modulation from theplurality of remote units arriving simultaneously at said base station,comprising the steps of:transmitting repetitively from said base stationan access signal having spread-spectrum modulation; receiving the accesssignal at said access-remote unit; transmitting, responsive to theaccess signal, from said accessing-remote unit, an echo signal havingspread-spectrum modulation; receiving the echo signal at said basestation; measuring a time delay at said base station between the accesssignal and the received echo signal; transmitting a protocol signalhaving spread-spectrum modulation from said base station to saidaccessing-remote unit, with the protocol signal communicating a chipcodeword and the time delay measured at said base station; receiving theprotocol signal at said accessing-remote unit; adjusting at saidaccessing-remote unit a delay time such that the communications signaltransmitted from said accessing-remote unit arrives simultaneously atsaid base station with communications signals transmitted from theplurality of remote units; and communicating from said base station tosaid plurality of remote units, with a plurality of base-communicationssignals having spread-spectrum modulation transmitted simultaneouslyfrom said base station.
 3. A method for establishing synchronous, codedivision multiple access communications between a base station and aplurality of remote units, with a plurality of remote-communicationssignals having spread-spectrum modulation from the plurality of remoteunits arriving simultaneously at said base station, the method for eachaccessing-remote unit comprising the steps of:transmitting repetitivelyfrom said base station an access signal having spread-spectrummodulation; receiving the access signal at said accessing-remote unit;transmitting responsive to the access signal, from said accessing-remoteunit, an echo signal having spread-spectrum modulation; receiving theecho signal at said base station; measuring a time delay at said basestation between the access signal and the received echo signal;transmitting a protocol signal having spread-spectrum modulation fromsaid base station to said accessing-remote unit, with the protocolsignal communicating the time delay measured at said base station;receiving the protocol signal at said accessing-remote unit; andadjusting at said accessing-remote unit a delay time such that thecommunications signal transmitted from said accessing-remote unitarrives simultaneously at said base station with communications signalstransmitted from the plurality of remote units.
 4. A method forestablishing and communicating synchronous, code division multipleaccess communications on a same frequency between a base station and aplurality of remote units, with a plurality of remote-communicationsignals having spread-spectrum modulation from the plurality of remoteunits arriving simultaneously at said base station, the method for eachaccessing-remote unit comprising the steps of:transmitting repetitivelyfrom said base station an access signal having spread-spectrummodulation; receiving the access signal at said access-remote unit;transmitting, responsive to the access signal, from saidaccessing-remote unit, an echo signal having spread-spectrum modulation;receiving the echo signal at said base station; measuring a time delayat said base station between the access signal and the received echosignal; transmitting a protocol signal having spread-spectrum modulationfrom said base station to said accessing-remote unit, with the protocolsignal communicating a chip codeword and the time delay measured at saidbase station; receiving the protocol signal at said accessing-remoteunit; adjusting at said accessing-remote unit a delay time such that thecommunications signal transmitted from said accessing-remote unitarrives simultaneously at said base station with communications signalstransmitted from the plurality of remote units; communicating from saidbase station to said plurality of remote units, with a plurality ofbase-communication signals having spread-spectrum modulation transmittedsimultaneously from said base station; and communicating from saidplurality of remote units to said base station with the plurality ofremote-communication signals having spread-spectrum modulationtransmitted from said plurality of remote units, respectively, theplurality of remote-communication signals arriving simultaneously atsaid base station.
 5. A method for communicating synchronous, codedivision multiple access communications on a same frequency between abase station and a plurality of remote units, the method comprising thesteps of:transmitting repetitively from said base station an accesssignal having spread-spectrum modulation; receiving the access signal atsaid access-remote unit; transmitting, responsive to the access signal,from said accessing-remote unit, an echo signal having spread-spectrummodulation; receiving the echo signal at said base station; measuring atime delay at said base station between the access signal and thereceived echo signal; transmitting a protocol signal havingspread-spectrum modulation from said base station to saidaccessing-remote unit, with the protocol signal communicating a chipcodeword and the time delay measured at said base station; receiving theprotocol signal at said accessing-remote unit; adjusting at saidaccessing-remote unit a delay time such that the communications signaltransmitted from said accessing-remote unit arrives simultaneously atsaid base station with communications signals transmitted from theplurality of remote units; and communicating from said plurality ofremote units to said base station with the plurality ofremote-communication signals having spread-spectrum modulationtransmitted from said plurality of remote units, respectively, theplurality of remote-communication signals arriving simultaneously atsaid base station.
 6. A method for establishing and communicatingsynchronous, code division multiple access communications on a samefrequency between a base station and a plurality of remote units, with aplurality of remote-communication signals having spread-spectrummodulation from the plurality of remote units arriving simultaneously atsaid base station, the method for each accessing-remote unit comprisingthe steps of:transmitting an access signal; receiving the access signal;transmitting an echo signal; receiving the echo signal; measuring a timedelay between the access signal and the received echo signal;transmitting a protocol signal; receiving the protocol signal; andadjusting a delay time.
 7. The method as set forth in claim 6, furthercomprising the step of:communicating from said plurality of remote unitsto said base station with the plurality of remote-communication signalshaving spread-spectrum modulation transmitted from said plurality ofremote units, respectively, the plurality of remote-communicationsignals arriving simultaneously at said base station.
 8. The method asset forth in claim 6, further comprising the step of:communicating fromsaid base station to said plurality of remote units.
 9. The method asset forth in claim 7, further comprising the step of:communicating fromsaid base station to said plurality of remote units.
 10. A system forcommunicating synchronous, code division multiple access communicationson a same frequency between a base station and a plurality of remoteunits, with a plurality of remote-communication signals havingspread-spectrum modulation from the plurality of remote units arrivingsimultaneously at said base station, the system comprising:a basestation for transmitting simultaneously a plurality ofbase-communication signals having spread-spectrum modulation; and aplurality of remote units for transmitting simultaneously a plurality ofremote-communication signals having spread-spectrum modulation; whereineach remote unit receives an access signal, each remote unit transmitsan echo signal having spread-spectrum modulation, each remote unitreceives a protocol signal, each remote unit adjusts a delay time suchthat the remote-communication signals transmitted from each remote unitarrives simultaneously at said base station with theremote-communication signals transmitted from the plurality of remoteunits.
 11. A system for establishing and communicating synchronous, codedivision multiple access communications on a same frequency between abase station and a plurality of remote units, with a plurality ofremote-communication signals having spread-spectrum modulation from theplurality of remote units arriving simultaneously at said base station,the system comprising:a base station, said base station transmittingrepetitively an access signal having spread-spectrum modulation, saidbase station receiving an echo signal having spread-spectrum modulation,said base station measuring a time delay between the access signal andthe received echo signal, said base station transmitting a protocolsignal having spread-spectrum modulation; and a plurality of remoteunits, each remote unit receiving the access signal, each remote unittransmitting the echo signal having spread-spectrum modulation, eachremote unit receiving the protocol signal, each remote unit adjusting adelay time such that the remote-communication signal transmitted fromeach remote unit arrives simultaneously at said base station with theremote-communication signals transmitted from the plurality of remoteunits.
 12. A system for communicating synchronous, code divisionmultiple access communications on a same frequency between a basestation and a plurality of remote units, comprising:a base station, saidbase station transmitting repetitively an access signal havingspread-spectrum modulation, said base station receiving an echo signalhaving spread-spectrum modulation, said base station measuring a timedelay between the access signal and the received echo signal, said basestation transmitting a protocol signal having spread-spectrummodulation; and a plurality of remote-units; wherein said plurality ofremote units communicate to said base station with a plurality ofremote-communication signals having spread-spectrum modulationtransmitted from said plurality of remote units, respectively, theplurality of remote-communication signals arriving simultaneously atsaid base station.
 13. The system as set forth in claim 12, wherein saidbase station communicates to said plurality of remote units, with aplurality of base-communication signals having spread-spectrummodulation transmitted simultaneously from said base station.